
<HTML>

<HEAD>

<TITLE>8086 assembler tutorial for beginners (part 8)</TITLE>

<META name="description" content="Procedures - Tutorial for Beginners">

</HEAD>

<BODY bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#0000FF" alink="#FF0000">





<FONT FACE="Verdana" SIZE=3>


<FONT SIZE=+1>
<B>8086 assembler tutorial for beginners (part 8)</B>
</FONT>


<BR><BR>

<FONT SIZE=+2><B>Procedures</B></FONT>
<BR><BR>

Procedure is a part of code that can be called from your
program in order to make some specific task. Procedures
make program more structural and easier to understand.
Generally procedure returns to the same point from where it was
called.
<BR><BR>
 The syntax for procedure declaration:
 <BLOCKQUOTE>
 <FONT FACE="Fixedsys">
 <U>name</U> PROC<BR><BR>
  &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;  ; here goes the code<BR>
  &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;  ;  of the procedure ...<BR><BR>
 RET<BR>
 <U>name</U> ENDP

 </FONT>
 </BLOCKQUOTE>

<BR>
<U>name</U> - is the procedure name, the same name should be in the top and the bottom,
              this is used to check correct closing of procedures.

<BR><BR>
Probably, you already know that <B>RET</B> instruction is used to return
to operating system. The same instruction is used to return from procedure
(actually operating system sees your program as a special procedure).

<BR><BR>
<B>PROC</B> and <B>ENDP</B> are compiler directives, so they are not assembled
into any real machine code. Compiler just remembers the address of procedure.

<BR><BR>
<B>CALL</B> instruction is used to call a procedure.

<BR><BR>
Here is an example:
<BR><BR>

<TABLE BORDER=1 CELLPADDING=10 WIDTH=50%><TR><TD>
<PRE><FONT FACE="Fixedsys">
ORG    100h

CALL   m1

MOV    AX, 2

RET                   ; return to operating system.

m1     PROC
MOV    BX, 5
RET                   ; return to caller.
m1     ENDP

END</FONT></PRE>
</TD></TR></TABLE>

<BR><BR>

The above example calls procedure <B>m1</B>, does <B>MOV BX, 5</B>, and returns
to the next instruction after <B>CALL</B>: <B>MOV AX, 2</B>.

<BR><BR>
There are several ways to pass parameters to procedure, the easiest way to
pass parameters is by using registers, here is another example of a procedure
that receives two parameters in <B>AL</B> and <B>BL</B> registers, multiplies
these parameters and returns the result in <B>AX</B> register:
<BR><BR>

<TABLE BORDER=1 CELLPADDING=10 WIDTH=50%><TR><TD>
<PRE><FONT FACE="Fixedsys">
ORG    100h

MOV    AL, 1
MOV    BL, 2

CALL   m2
CALL   m2
CALL   m2
CALL   m2

RET                   ; return to operating system.

m2     PROC
MUL    BL             ; AX = AL * BL.
RET                   ; return to caller.
m2     ENDP

END</FONT></PRE>
</TD></TR></TABLE>

<BR><BR>
In the above example value of <B>AL</B> register is update every time
the procedure is called, <B>BL</B> register stays unchanged, so this
algorithm calculates <B>2</B> in power of <B>4</B>,<BR>
so final result in <B>AX</B> register is <B>16</B> (or 10h).

<BR><BR>
<HR>
<BR>

Here goes another example,<BR> that uses a procedure
to print a <I>Hello World!</I> message:
<BR><BR>

<TABLE BORDER=1 CELLPADDING=10 WIDTH=50%><TR><TD>
<PRE><FONT FACE="Fixedsys">
ORG    100h

LEA    SI, msg        ; load address of msg to SI.

CALL   print_me

RET                   ; return to operating system.

; ==========================================================
; this procedure prints a string, the string should be null
; terminated (have zero in the end),
; the string address should be in SI register:
print_me     PROC

next_char:
    CMP  b.[SI], 0    ; check for zero to stop
    JE   stop         ;

    MOV  AL, [SI]     ; next get ASCII char.

    MOV  AH, 0Eh      ; teletype function number.
    INT  10h          ; using interrupt to print a char in AL.

    ADD  SI, 1        ; advance index of string array.

    JMP  next_char    ; go back, and type another char.

stop:
RET                   ; return to caller.
print_me     ENDP
; ==========================================================

msg    DB  'Hello World!', 0   ; null terminated string.

END</FONT></PRE>
</TD></TR></TABLE>

<BR><BR>
"<B>b.</B>" - prefix before [SI] means that we need to compare bytes,
not words. When you need to compare words add "<B>w.</B>" prefix instead.
When one of the compared operands is a register it's not required because
compiler knows the size of each register.

<BR><BR><BR>

<HR>
<CENTER>
<A HREF="asm_tutorial_07.html"><B> &lt;&lt;&lt; previous part &lt;&lt;&lt; </B></A>
&nbsp;&nbsp;&nbsp;&nbsp;
<A HREF="asm_tutorial_09.html"><B> &gt;&gt;&gt; Next Part &gt;&gt;&gt; </B></A>
</CENTER>
<HR>


<BR>

</FONT>




</BODY>

</HTML>
